Principles of Inheritance and Variation

• ABO blood group gene I, with three alleles: Iᵀ, Iᴿ and i.
• Phenotypes: A, B, AB and O — four.
• Genotypes: IᵀIᵀ, Iᵀi, IᴿIᴿ, Iᴿi, IᵀIᴿ, ii — six.

The truth of each statement and whether R explains A.

Both statements are true. The ABO system is controlled by a single gene with multiple alleles, and it does give four phenotypes and six genotypes.

But the counting in R does not explain the claim in A. Knowing how many phenotypes exist does not tell you the trait is governed by one gene — the numbers are a consequence of the single-gene, three-allele arrangement, not a reason for it.

1. Assertion. ABO is a classic example of multiple allelism at a single locus. True.
2. Reason. Six genotypes and four phenotypes is exactly right. True.
3. Does R explain A? No — the counts follow from the single gene rather than establishing it.
4. Both true, but R is not the explanation.

• Overall masculine development — so the individual is male.
• Gynaecomastia — development of breast tissue.
• Sterile.

Which disorder this is.

The word masculine settles it immediately: Turner's syndrome individuals are female (45, XO), so it cannot be Turner's.

Klinefelter's syndrome is 47, XXY — a male with an extra X. The additional X produces some feminine development including gynaecomastia, and the individual is sterile.

1. The individual is described as masculine, so look for a male karyotype.
2. Turner's is 45, XO — female. Eliminated on the first word of the stem.
3. Klinefelter's is 47, XXY — male, with an extra X.
4. The extra X causes gynaecomastia and sterility.
5. Klinefelter's syndrome.

• Non-disjunction: failure of chromosomes to separate during meiosis.
• Autosomal: involving a non-sex chromosome.
• Primary: occurring in the first meiotic division.

Which disorder results from autosomal non-disjunction.

Two words filter the options. Non-disjunction produces a chromosome number abnormality, which rules out sickle-cell anaemia — that is a single base substitution with a perfectly normal karyotype.

Autosomal then rules out Klinefelter's and Turner's, which both involve the sex chromosomes. That leaves Down's syndrome, trisomy of chromosome 21.

1. Sickle-cell anaemia: a point mutation (Glu → Val), not a chromosome count problem. Eliminated.
2. Klinefelter's (47, XXY): non-disjunction, but of the sex chromosomes. Eliminated by the word autosomal.
3. Turner's (45, XO): same objection.
4. Down's syndrome: trisomy of chromosome 21, an autosome. Correct.

• A three-generation pedigree with filled symbols marking affected individuals.
• Affected individuals appear in generation I and again in generation III.
• Generation II shows no affected individuals.
• Both males and females are affected.

The mode of inheritance.

The decisive feature is that the trait skips a generation: unaffected parents in generation II produce affected children in generation III. That is only possible if both parents were carriers — which means the allele is recessive.

Since both sexes are affected roughly equally, it is autosomal rather than X-linked.

1. Does it skip? Yes — generation II is unaffected but generation III is not.
2. A dominant trait cannot skip: every affected child must have an affected parent. So dominant is eliminated.
3. Therefore the allele is recessive, carried unseen through generation II.
4. Autosomal or X-linked? Affected individuals include both sexes in comparable numbers, so it is autosomal.
5. Autosomal recessive.

• Two statements about why Drosophila suited Morgan's work.

The truth of each statement.

Both statements alter one detail of an otherwise correct sentence, which is what makes them hard.

Statement I is right about the two-week life cycle but wrong about “a few” progeny — a single mating produces a large number, and that abundance is precisely why Drosophila was chosen.

Statement II is right that there are many hereditary variations, but NCERT specifies they are seen with a low-power microscope, not the naked eye.

1. Statement I. The life cycle and the fly are described correctly, but a single mating gives a large number of progeny, not a few. Incorrect.
2. Statement II. The variations are real, but they are observed under a low-power microscope. Incorrect.
3. Both statements are incorrect.

• Assertion about independent assortment and linkage.
• Reason attributing an observation about linkage to Mendel.

The truth of each statement.

The assertion is true. Independent assortment requires the two genes to be on different chromosomes. Completely linked genes sit on the same chromosome with no crossing over between them, so they always travel together and the law fails.

The reason is false on attribution. Mendel never observed linkage — that was Morgan, working with Drosophila decades later. Mendel's seven pea characters happened to behave independently.

1. Assertion. Complete linkage means the alleles never separate, so they cannot assort independently. Correct.
2. Reason. The claim itself — no recombinants under complete linkage — is true, but it was Morgan who observed it, not Mendel. Incorrect.
3. (A) is correct, but (R) is incorrect.

• Gene B in pea, affecting starch grains.
• BB gives large starch grains; bb gives small ones.
• The phenotype being measured is starch grain size.

What kind of dominance Bb shows for this phenotype.

The heterozygote Bb produces starch grains of intermediate size — larger than bb, smaller than BB. An intermediate heterozygote is the definition of incomplete dominance.

The important subtlety: judged by seed shape, Bb peas are fully round like BB, so the same gene shows complete dominance on that phenotype. Dominance is a property of the phenotype you choose to measure, not of the gene alone.

1. BB seeds have large starch grains; bb seeds have small ones.
2. Bb seeds have grains of intermediate size — a blend.
3. An intermediate heterozygote means incomplete dominance.
4. Note the contrast: for seed shape, Bb is fully round like BB, so the same gene shows complete dominance there.

• Father is colourblind: XᵇY.
• Mother is normal with no family history, so she is homozygous normal: XᵄXᵄ.
• Colour blindness is X-linked recessive.

The condition of their sons.

A son receives his Y from his father and his X from his mother. The father's affected X therefore never reaches a son — it goes only to daughters.

Since the mother carries no defective allele at all, every son gets a normal X and is normal-visioned.

1. Father XᵇY gives either Xᵇ or Y.
2. Mother XᵄXᵄ can only give Xᵄ.
3. Sons = mother's Xᵄ + father's Y = XᵄY — all normal.
4. Daughters = mother's Xᵄ + father's Xᵇ = XᵄXᵇ — all carriers, none affected.
5. All sons are normal-visioned.

• The woman has two X chromosomes, one carrying the condition.
• She passes one X to each child, chosen at random.
• Sons receive X from mother + Y from father; daughters receive X from each parent.

Which children can inherit that particular X chromosome.

A mother passes one of her two X chromosomes to every child, whatever its sex. So the affected X has a 50% chance of reaching any child — son or daughter.

The father's contribution differs by sex (X to daughters, Y to sons), but that has no bearing on which of the mother's X chromosomes is passed on.

1. The mother has two X chromosomes and passes one at random to each child.
2. A son gets that X plus a Y from his father.
3. A daughter gets that X plus an X from her father.
4. Either way, the affected X can be inherited.
5. Both sons and daughters.

• Pink flowers in snapdragon — incomplete dominance.
• “B” blood group in humans — multiple alleles.
• White flowers in pea — recessive trait.
• White-eyed fruit flies — X-linked recessive trait.

The one case where the genotype cannot be predicted from the phenotype.

The genotype is predictable from the phenotype whenever the phenotype corresponds to exactly one genotype. That is true for any recessive phenotype (it must be homozygous) and for any incomplete dominance heterozygote (it has its own distinct appearance).

It fails for a dominant phenotype in a system with multiple alleles: blood group B can be IᴿIᴿ or Iᴿi, and the two are indistinguishable by appearance.

1. Pink snapdragon: incomplete dominance, so pink is only Rr. Predictable.
2. White pea flowers: recessive, so it must be homozygous. Predictable.
3. White-eyed fruit flies: X-linked recessive, so females are XᵇXᵇ and males XᵇY. Predictable.
4. Blood group B: could be IᴿIᴿ or Iᴿi — two genotypes, one phenotype. Not predictable.